Method and apparatus for treating a hydrocarbon stream

ABSTRACT

The present invention relates to a method of treating a hydrocarbon stream, the method at least comprising the steps of: supplying a partly condensed feed stream ( 10 ). to a first gas/liquid separator ( 2 ) and into a gaseous stream ( 2  and a liquid stream ( 30 ); expanding the liquid stream ( 30 ) and the gaseous stream ( 20 ) and subsequently feeding them into a second gas/liquid separator ( 3 ) at a first and second feeding point ( 32 ) respectively, the second feeding point ( 32 ) being at a higher level than the first feeding point ( 31 ); feeding a liquefied natural gas stream ( 70,70   b ) into the second gas/liquid separator ( 3 ) at a third feeding point ( 33 ) being at a higher level than the second feeding point ( 32 ); removing from the top of the second gas/liquid separator ( 3 ) a C2+ lean gaseous stream ( 60 ) and from the bottom a liquid stream ( 80, 80   a ); wherein the liquefied ̂natural gas stream ( 70,70   b ) is obtained from a source ( 4 ) of liquefied natural gas from a separate plant.

The present invention relates to a method and/or apparatus of treating ahydrocarbon stream such as a natural gas stream.

Several processes and apparatuses for treating a hydrocarbon stream areknown. An example is given in US 2005/0268649 A1 relating to a processfor processing natural gas or other methane-rich gas streams to producea liquefied natural gas (LNG) stream that has a high methane purity anda liquid stream containing predominantly hydrocarbons heavier thanmethane.

A problem of the known method is that it is rather complicated therebyresulting in high capital expenses (CAPEX), but at the same time it doesnot obtain a satisfactory recovery of in particular ethane. Also theamount of fuel gas produced is not optimal, because the focus of US2005/0268649 A1 is mainly on the liquefaction of natural gas rather thanon the production of ‘residual streams’ and recovery of ethanetherefrom.

A further problem of US 2005/0268649 A1 is that the start up of themethod or the plant for performing the method takes a significant amountof time.

It is an object of the present invention to minimize one or more of theabove problems, while at the same time maintaining or even improving therecovery of ethane and heavier hydrocarbons, in particular ethane, fromthe hydrocarbon stream.

It is a further object of the present invention to provide analternative method for treating a natural gas stream.

The present invention provides a method of treating a hydrocarbon streamsuch as a natural gas stream, in a plant, the method at least comprisingthe steps of:

(a) supplying a partly condensed feed stream to a first gas/liquidseparator;

(b) separating the feed stream in the first gas/liquid separator into agaseous stream and a liquid stream;

(c) expanding the liquid stream obtained in step (b) and feeding it intoa second gas/liquid separator at a first feeding point;

(d) expanding the gaseous stream obtained in step (b), thereby obtainingan at least partially condensed stream, and subsequently feeding it intothe second gas/liquid separator at a second feeding point, the secondfeeding point being at a higher level than the first feeding point;

(e) feeding a liquefied natural gas stream into the second gas/liquidseparator at a third feeding point, the third feeding point being at ahigher level than the second feeding point;

(f) removing from the top of the second gas/liquid separator a C2+ leangaseous stream; and

(g) removing from the bottom of the second gas/liquid separator a liquidstream;

wherein the liquefied natural gas stream as fed in step (e) is obtainedfrom a source of liquefied natural gas from a separate plant.

In a further aspect the present invention relates to an apparatus fortreating a hydrocarbon stream such as a natural gas stream, in a plant,the apparatus at least comprising:

a first gas/liquid separator having an inlet for a partly condensed feedstream, a first outlet for a gaseous stream and a second outlet for aliquid stream;

a second gas/liquid separator having at least a first outlet for agaseous stream and a second outlet for a liquid stream and first, secondand third feeding points, the third feeding point being at a higherlevel in the gas/liquid separator than the second feeding point, saidsecond feeding point being at a higher level in the gas/liquid separatorthan the first feeding point;

a first expander for expanding the gaseous stream obtained from thefirst outlet of the first gas/liquid separator, the first expanderhaving an outlet which is connected to the second feeding point of thesecond gas/liquid separator; and

a second expander for expanding the liquid stream obtained from thesecond outlet of the first gas/liquid separator, the second expanderhaving an outlet which is connected to the first feeding point of thesecond gas/liquid separator;

wherein the third feeding point is connected to a source of liquefiednatural gas from a separate plant.

Hereinafter the invention will be further illustrated by way of exampleand with reference to the following non-limiting drawing. In thedrawing,

FIG. 1 schematically shows a process scheme in accordance with thepresent invention.

For the purpose of this description, a single reference number will beassigned to a line as well as a stream carried in that line. Samereference numbers refer to similar components.

Embodiments of the present invention relate to the treatment of anatural gas stream, and may involve recovery of at least some of theethane, propane, butanes and higher hydrocarbons such as pentane fromthe natural gas. The recovery of hydrocarbons may be done for severalpurposes. One purpose may be the production of hydrocarbon streamsconsisting primarily of hydrocarbon products heavier than methane suchas natural gas liquids (NGLs; usually composed of ethane, propane andbutanes), liquefied petroleum gas (LPG; usually composed of propane andbutane) or condensates (usually composed of butanes and heavierhydrocarbon components). Another purpose may be the adjustment of e.g.the heating value of the hydrocarbon stream to correspond to desiredspecifications of sales gas.

It has been found that, using the surprisingly simple methods set forthin the present disclosure, the CAPEX can be significantly lowered. As anexample no (partial) reflux of the gaseous overhead stream from thesecond gas/liquid separator is needed.

Further, also due to its simplicity, the methods and apparatusesdescribed herein are expected to be very robust and can be started upquickly when compared with known line-ups.

Furthermore it has been found a high ethane recovery may be obtainedusing the methods described herein. thereby resulting in a leanermethane-rich natural gas stream. The methods have also been contemplatedto be suitable for feed streams having a pressure well below 70 bar, atthe same time maintaining a relatively high ethane recovery.

Another advantage is that it is suitable for a broad range of feedstream compositions.

A further advantage is that more sales gas is produced, in particular ifthe overhead gas is not subsequently liquefied or refluxed to the secondgas/liquid separator (contrary to e.g. US 2005/0268649 A1).

The hydrocarbon feed stream may be any suitable hydrocarbon-containingstream to be treated, but is usually a natural gas stream which may beobtained from natural gas or petroleum reservoirs. As an alternative thenatural gas stream may also be obtained from another source, alsoincluding a synthetic source such as a Fischer-Tropsch process.

Usually the feed stream is comprised substantially of methane.Preferably the feed stream comprises at least 60 mole % methane, morepreferably at least 80 mole % methane.

Depending on the source, the feed stream may contain varying amounts ofhydrocarbons heavier than methane such as ethane, propane, butanes andpentanes as well as some aromatic hydrocarbons. The feed stream may alsocontain non-hydrocarbons such as H₂O, N₂, CO₂, H₂S and other compounds,and the like.

Embodiments of the invention involve supplying a feed stream to a firstgas/liquid separator; separating the feed stream into a gaseous streamand a liquid stream; and feeding these gaseous and liquid streams into asecond gas/liquid separator.

If desired, the feed stream containing the natural gas may bepre-treated before feeding it to the first gas/liquid separator. Thispre-treatment may comprise removal of undesired components such as CO₂and H₂S, or other steps such as pre-cooling, pre-pressurizing or thelike. As these steps are well known to the person skilled in the art,they are not further discussed here. Preferably the hydrocarbon feedstream contains < (less than) 1 mole % CO₂.

The first and second gas/liquid separator may be any suitable means forobtaining at least a gaseous stream and a liquid stream, such as ascrubber, distillation column, etc. If desired, three or more gas/liquidseparators may be present. Preferably the second gas/liquid separator isa de-methanizer, i.e. the overhead stream of the second gas/liquidseparator being methane-enriched and the bottom stream of the secondgas/liquid separator being ethane-enriched when compared with thehydrocarbon feed stream.

The gaseous and liquid streams may be expanded before feeding them intothe second gas/liquid separator. The person skilled in the art willunderstand that the steps of expanding may be performed in various waysusing any expansion device (e.g. using a throttling valve, a flash valveor a common expander).

Further, the person skilled in the art will readily understand thattreated hydrocarbon streams may be further processed, if desired. Also,further intermediate processing steps between the first and secondgas/liquid separator may be performed.

The invention provides methods and apparatuses wherein a C2+ leangaseous stream is produced in a plant, whereby a liquefied natural gas(LNG) stream is employed that is obtained from a source of liquefiednatural gas from a separate plant. In what may hereinafter be referredto as step (e), the LNG stream may be fed into the second gas/liquidseparator.

Preferably the LNG stream has a temperature below −100° C. and is atleast partially condensed and comprises more than 60 mole % methane,preferably more than 80 mole % methane. The phrase “source of liquefiednatural gas from a separate plant” means that the LNG is produced in aseparate plant from the plant in which the C2+ lean gaseous stream isproduced. No LNG stream is used that is generated in the hydrocarbontreating plant of the invention, such as any LNG stream produced duringthe treating itself or downstream of the treating (e.g. downstream ofthe second gas/liquid separator). Thus, contrary to e.g. US 2005/0268649A1, an already existing LNG stream is used that has been producedelsewhere, for example an LNG stream produced in a separate liquefactionplant. The separate source may be a storage tank or a stream from anearby liquefaction plant. Also, the LNG stream may have been obtainedfrom an offloading LNG carrier vessel. Preferably the LNG stream isobtained from a separate plant at an LNG import terminal, such as an LNGstorage tank at an LNG import terminal.

In a step (f), a gaseous stream is removed from the top of the secondgas/liquid separator. The gaseous stream obtained in step (f) is a C2+lean gaseous stream. By “C2+ lean” is meant a gaseous stream which has alow proportion of hydrocarbons having two or more carbon atoms,including a stream rich in methane, with only a minor proportion ofhydrocarbons having two or more carbon atoms. In this way, the gaseousstream obtained in step (f) is provided with a composition and/orheating value required for used as a sales gas. For instance, the C2+lean gaseous stream removed from the top of the second gas/liquidseparator in step (f) may comprise < (less than) 10 mole % hydrocarbonshaving 2 of more carbon atoms, preferably <5 mole %, more preferably <2mole % and even more preferably <1 mole %.

According to an embodiment described herein the gaseous stream obtainedin step (f) is sent to a gas network and not liquefied to obtain amethane-rich LNG stream.

Further it may be that the gaseous stream obtained in step (f) is heatexchanged against the feed stream.

Moreover, the LNG stream as fed in step (e) may have been previouslyheat exchanged against the feed stream.

In a step (g), a liquid stream is removed from the bottom of the secondgas/liquid separator. This liquid stream may be further processed.Preferably, during such further processing, this liquid stream issubjected to fractionation thereby obtaining two or more products,including ethane.

It is preferred that > (more than) 75 mole % of hydrocarbons having 2 ormore carbon atoms present in the partially condensed feed stream arerecovered in the liquid stream obtained from the second gas/liquidseparator, preferably >80, more preferably >85, even morepreferably >90, most preferably >95 mole %. Viewed another way, it ispreferred that ≦ (less than or equal to) 25 mole % of the hydrocarbonshaving 2 or more carbon atoms present in the partially condensed feedstream are transferred to the gaseous stream obtained in step (f),preferably ≦20, more preferably ≦15, even more preferably ≦10, mostpreferably ≦5 mole %.

For instance, it is preferred that > (more than) 75 mole % of the ethanepresent in the paritially condensed feed stream is recovered in theliquid stream obtained in step (g), preferably >80, more preferably >85,even more preferably >90, most preferably >95 mole %. Viewed anotherway, it is preferred that s (less than or equal to) 25 mole % of theethane present in the partially condensed feed stream is transferred tothe gaseous stream obtained from the second gas/liquid separator in step(f), preferably ≦20, more preferably ≦15, even more preferably ≦10, mostpreferably ≦5 mole %. According to an embodiment described herein thepressure drop during expanding—in a step (d)—of the gaseous streamobtained from the first gas/liquid separator is less than 15 bar, morepreferably less than 10 bar, even more preferably less than 5 bar.Herewith the energy consumption in the treating process can be reduced.

Also it is preferred that the pressure in the second gas/liquidseparator is from 15 to 40 bar, preferably from 20 to 30 bar. Examplesinclude from 23 to 27 bar and about 25 bar.

FIG. 1 schematically shows a process scheme (generally indicated withreference no. 1) for the treating of a hydrocarbon feed stream such asnatural gas whereby ethane and heavier hydrocarbons are recovered to acertain extent.

The process scheme of FIG. 1 comprises a first gas/liquid separator 2, asecond gas/liquid separator 3, a first expander 6, a second expander 7(in the from of a throttling valve), a separate source 4 of LNG from aseparate plant (in the embodiment of FIG. 1 an LNG storage tank at anLNG import terminal), a gas network 11 and a fractionation unit 9. Thesecond gas/liquid separator 3 may be provided in the form of adistillation column 3, such as a de-methanizer as is the case in theembodiment of present FIG. 1. The person skilled in the art will readilyunderstand that further elements may be present if desired.

During use, a partly condensed feed stream 10 containing natural gas issupplied to the inlet 21 of the first gas/liquid separator 2 at acertain inlet pressure and inlet temperature. Typically, the inletpressure to the first gas/liquid separator 2 will be between 10 and 100bar, preferably above 30 bar and preferably below 90 bar, morepreferably below 70 bar. The temperature will usually be between 0 and−80° C. To obtain the partly condensed feed stream 10, it may have beenpre-cooled in several ways. In the embodiment of FIG. 1, the feed steamhas been heat exchanged in heat exchanger 12 against a C2+ lean gaseousstream 60 (hereinafter also referred to as gaseous overhead stream 60 orjust overhead stream 60, and to be discussed hereafter) and subsequentlyin heat exchanger 5 against a LNG stream, in the embodiment of FIG. 1 anLNG stream 70 (indicated as streams 70 a and 70 b) originating from theseparate plant. Instead of or in addition to heat exchange againststreams 60 and 70 also a common external refrigerant such as propane oranother cooler such as an air or water cooler may be used to cool thefeed stream 10.

If desired the feed stream 10 may have been further pre-treated beforeit is fed to the first gas/liquid separator 2. As an example, H₂O, CO₂,H₂S and hydrocarbon components having the molecular weight of pentane orhigher may also at least partially have been removed from the feedstream 10 before entering the first separator 2.

In the first gas/liquid separator 2, the feed stream 10 (fed at inlet21) is separated into a gaseous stream 20 (removed at first outlet 22)and a liquid stream 30 (removed at second outlet 23). The gaseous stream20, which may hereinafter be referred to as the gaseous overhead stream20, is enriched in methane relative to the feed stream 10.

The liquid stream 30, which may hereinafter be referred to as the liquidbottom stream 30, is generally liquid and usually contains somecomponents that are freezable when they would be brought to atemperature at which methane is liquefied. The bottom stream 30 may alsocontain hydrocarbons that can be separately processed to form liquefiedpetroleum gas (LPG) products. The stream 30 is expanded in the secondexpander 7 to the operating pressure of the distillation column 3(usually about 25 bar) and fed into the same at a first feeding point 31as stream 40. If desired a further heat exchanger (not shown) may bepresent on line 40 to heat the stream 40. The second expander 7 may beany expansion device such as a liquid expander as well as a flash valve.

The gaseous overhead stream 20 removed at the first outlet 22 of thefirst separator 2 is at least partially condensed in the first expander6 and subsequently fed as an at least partially condensed stream 50 intothe distillation column 3 at a second feeding point 32. The secondfeeding point 32 is at a higher level than the first feeding point 31.If desired a further heat exchanging step may take place between thefirst expander 6 and the second feeding point 32. The pressure drop overthe expander 6 may be lower than 15 bar, even lower than 10 bar, as noextra cooling is required for the stream 50 in view of the use of thecold stream 70 (to be discussed hereafter).

If desired (and as indicated with dashed lines in FIG. 1) the gaseousoverhead stream 20 may be split into two streams; the ‘additional’stream 20 a may be expanded in expander 6 a and fed into thedistillation column at a further feeding point 37.

The LNG stream 70 is, after cooling the feed stream 10 in heat exchanger5, fed as stream 70 b into the distillation column 3 at a third feedingpoint 33, the third feeding point 33 being at a higher level than thesecond feeding point 32. Preferably the third feeding point 33 is at ornear the top of the distillation column 3.

Preferably, the pressure in the distillation column 3 is from 15 to 40bar, preferably from 20 to 30 bar.

Preferably, the temperature of the LNG stream 70 is below −150° C. justbefore heat exchanging (as stream 70 a) against stream 10 in heatexchanger 5, and below −100° C. but usually above −150° C. just beforefeeding (as stream 70 b) in the second gas/liquid separator 3 at thethird feeding point 33. Herewith no reflux of the overhead stream 60 tothe distillation column 3 is required in order to recover a major partof the ethane present in the feed stream 10 in the bottom product stream80. As a result the capital expenses are significantly reduced.

The gaseous overhead stream 60 obtained at the top of the secondgas/liquid separator 3 (at first outlet 34) is sent to the gas network11 (for use as a sales gas) after heat exchanging against the feedstream 10 in heat exchanger 12 and optionally compressing in compressor8 (which may be functionally coupled to first expander 6). Preferably,the gaseous stream 60 is not subsequently liquefied.

A liquid stream 80, hereinafter also referred to as a liquid bottomstream 80, is removed from the second outlet 35 of the distillationcolumn 3. Liquid bottom stream 80 may be cooled in ambient cooler 81 andis usually subjected to one or more fractionation steps, e.g. in afractionation unit 9, to collect various natural gas liquid products.For example, as shown in FIG. 1, fractionation unit 9 may produce atleast two liquid streams (100, 110), such as a liquefied petroleum gas(LPG) stream and a condensate stream. Usually an ethane stream (notshown) is also produced from fractionation unit 9.

If desired, and as shown in FIG. 1, a part of the liquid bottom stream80 may be returned to the bottom of the distillation column 3 as stream90, the remainder of stream 80 being indicated with stream 80 a.

The person skilled in the art will understand that the amount of ethanerecovered in the bottom stream will also be dependent on the compositionof the LNG stream 70 originating from the source 4. In case the LNGstream 70 contains large amounts of ethane, this ethane will besubstantially recovered in the bottom stream 80.

Table I gives an overview of the pressures and temperatures of a streamat various parts in an example process of FIG. 1. The feed stream inline 10 of FIG. 1 comprised approximately the following composition: 79mole % methane, 10 mole % ethane, 6 mole % propane, 3 mole % butanes andpentane, and 2 mole % N₂. Other components such as H₂S and H₂O werepreviously removed.

TABLE I Temperature Mole % Line Pressure (bar) (° C.) ethane 10 35.5−70.0 10.0 20 35.4 −69.6 3.4 30 35.4 −69.6 20.8 40 20.2 −83.7 20.8 5020.2 −91.6 3.4 60 20.0 −104.7 0.4 70a 20.0 −155 8.5 80 20.2 −19.0 51.4

It was found that according to the present invention the amount ofoverhead gas stream 60 (that can be used as sales gas) was relativelyhigh when compared with the same line-up as FIG. 1, but wherein a refluxcolumn was used for the overhead stream 60 as a result of which a partof the stream 60 was refluxed to the distillation column 3.

The person skilled in the art will readily understand that manymodifications may be made without departing from the scope of theinvention. As an example, the compressor may comprise two or morecompression stages. Further, each heat exchanger may comprise a train ofheat exchangers.

1. Method of treating a hydrocarbon stream in a plant, the method atleast comprising the steps of: (a) supplying a partly condensed feedstream to a first gas/liquid separator; (b) separating the feed streamin the first gas/liquid separator into a gaseous stream and a liquidstream; (c) expanding the liquid stream obtained in step (b) and feedingit into a second gas/liquid separator at a first feeding point; (d)expanding the gaseous stream obtained in step (b), thereby obtaining anat least partially condensed stream, and subsequently feeding it intothe second gas/liquid separator at a second feeding point, the secondfeeding point being at a higher level than the first feeding point; (e)feeding a liquefied natural gas stream into the second gas/liquidseparator at a third feeding point, the third feeding point being at ahigher level than the second feeding point; (f) removing from the top ofthe second gas/liquid separator a C2+ lean gaseous stream; and (g)removing from the bottom of the second gas/liquid separator a liquidstream; wherein the liquefied natural gas stream as fed in step (e) isobtained from a source of liquefied natural gas from a separate plant.2. Method according to claim 1, wherein the liquefied natural gas streamhas a temperature below −100° C.
 3. Method according to claim 1, whereinthe gaseous stream obtained in step (f) is sent to a gas network. 4.Method according to claim 1, wherein the gaseous stream obtained in step(f) is not liquefied.
 5. Method according to claim 1, wherein thegaseous stream obtained in step (f) is heat exchanged against the feedstream.
 6. Method according to claim 1, wherein the liquefied naturalgas stream as fed in step (e) has been previously heat exchanged againstthe feed stream.
 7. Method according to claim 1, wherein the liquidstream removed from the bottom of the second gas/liquid separator issubjected to fractionation thereby obtaining two or more productsincluding ethane.
 8. Method according to claim 1, wherein >75 mole % ofthe ethane present in the partially partly condensed feed stream isrecovered in the liquid stream obtained in step (g).
 9. Method accordingto claim 1, wherein the pressure drop during expanding in step (d) isless than 15 bar.
 10. Method according to claim 1, wherein the pressurein the second gas/liquid separator is from 15 to 40 bar.
 11. Methodaccording to claim 1, wherein the C2+ lean gaseous stream removed fromthe top of the second gas/liquid separator in step (f) is a gaseousstream comprising < (less than) 10 mole % hydrocarbons having 2 or morecarbon atoms.
 12. Method according to claim 1, wherein the source ofliquefied natural gas from a separate plant is a liquefied natural gasstorage tank at a liquefied natural gas import terminal.
 13. Apparatusfor treating a hydrocarbon stream in a plant, the apparatus at leastcomprising: a first gas/liquid separator having an inlet for a partlycondensed feed stream, a first outlet for a gaseous stream and a secondoutlet for a liquid stream; a second gas/liquid separator having atleast a first outlet for a gaseous stream and a second outlet for aliquid stream and first, second and third feeding points, the thirdfeeding point being at a higher level in the gas/liquid separator thanthe second feeding point, said second feeding point being at a higherlevel in the gas/liquid separator than the first feeding point; a firstexpander for expanding the gaseous stream obtained from the first outletof the first gas/liquid separator, the first expander having an outletwhich is connected to the second feeding point of the second gas/liquidseparator; and a second expander for expanding the liquid streamobtained from the second outlet of the first gas/liquid separator, thesecond expander having an outlet which is connected to the first feedingpoint of the second gas/liquid separator; wherein the third feedingpoint is connected to a source of liquefied natural gas from a separateplant.
 14. Apparatus according to claim 13 wherein the source ofliquefied natural gas from a separate plant is a liquefied natural gasstorage tank at a liquefied natural gas import terminal.
 15. Methodaccording to claim 2, wherein the gaseous stream obtained in step (f) issent to a gas network.
 16. Method according to claim 2, wherein thegaseous stream obtained in step (f) is not liquefied.
 17. Methodaccording to claim 3, wherein the gaseous stream obtained in step (f) isnot liquefied.
 18. Method according to claim 2, wherein the gaseousstream obtained in step (f) is heat exchanged against the feed stream.19. Method according to claim 3, wherein the gaseous stream obtained instep (f) is heat exchanged against the feed stream.
 20. Method accordingto claim 4, wherein the gaseous stream obtained in step (f) is heatexchanged against the feed stream.